ACK: [PATCH Xenial/Yakkety SRU] percpu-refcount: fix reference leak during percpu-atomic transition

[Colin Ian King]

On 08/02/17 20:33, Tim Gardner wrote:
> From: Douglas Miller <dougmill at linux.vnet.ibm.com>
> 
> BugLink: http://bugs.launchpad.net/bugs/1662673
> 
> percpu_ref_tryget() and percpu_ref_tryget_live() should return
> "true" IFF they acquire a reference. But the return value from
> atomic_long_inc_not_zero() is a long and may have high bits set,
> e.g. PERCPU_COUNT_BIAS, and the return value of the tryget routines
> is bool so the reference may actually be acquired but the routines
> return "false" which results in a reference leak since the caller
> assumes it does not need to do a corresponding percpu_ref_put().
> 
> This was seen when performing CPU hotplug during I/O, as hangs in
> blk_mq_freeze_queue_wait where percpu_ref_kill (blk_mq_freeze_queue_start)
> raced with percpu_ref_tryget (blk_mq_timeout_work).
> Sample stack trace:
> 
> __switch_to+0x2c0/0x450
> __schedule+0x2f8/0x970
> schedule+0x48/0xc0
> blk_mq_freeze_queue_wait+0x94/0x120
> blk_mq_queue_reinit_work+0xb8/0x180
> blk_mq_queue_reinit_prepare+0x84/0xa0
> cpuhp_invoke_callback+0x17c/0x600
> cpuhp_up_callbacks+0x58/0x150
> _cpu_up+0xf0/0x1c0
> do_cpu_up+0x120/0x150
> cpu_subsys_online+0x64/0xe0
> device_online+0xb4/0x120
> online_store+0xb4/0xc0
> dev_attr_store+0x68/0xa0
> sysfs_kf_write+0x80/0xb0
> kernfs_fop_write+0x17c/0x250
> __vfs_write+0x6c/0x1e0
> vfs_write+0xd0/0x270
> SyS_write+0x6c/0x110
> system_call+0x38/0xe0
> 
> Examination of the queue showed a single reference (no PERCPU_COUNT_BIAS,
> and __PERCPU_REF_DEAD, __PERCPU_REF_ATOMIC set) and no requests.
> However, conditions at the time of the race are count of PERCPU_COUNT_BIAS + 0
> and __PERCPU_REF_DEAD and __PERCPU_REF_ATOMIC set.
> 
> The fix is to make the tryget routines use an actual boolean internally instead
> of the atomic long result truncated to a int.
> 
> Fixes: e625305b3907 percpu-refcount: make percpu_ref based on longs instead of ints
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=190751
> Signed-off-by: Douglas Miller <dougmill at linux.vnet.ibm.com>
> Reviewed-by: Jens Axboe <axboe at fb.com>
> Signed-off-by: Tejun Heo <tj at kernel.org>
> Fixes: e625305b3907 ("percpu-refcount: make percpu_ref based on longs instead of ints")
> Cc: stable at vger.kernel.org # v3.18+
> (cherry picked from commit 966d2b04e070bc040319aaebfec09e0144dc3341)
> Signed-off-by: Tim Gardner <tim.gardner at canonical.com>
> ---
>  include/linux/percpu-refcount.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h
> index 1c7eec0..3a481a4 100644
> --- a/include/linux/percpu-refcount.h
> +++ b/include/linux/percpu-refcount.h
> @@ -204,7 +204,7 @@ static inline void percpu_ref_get(struct percpu_ref *ref)
>  static inline bool percpu_ref_tryget(struct percpu_ref *ref)
>  {
>  	unsigned long __percpu *percpu_count;
> -	int ret;
> +	bool ret;
>  
>  	rcu_read_lock_sched();
>  
> @@ -238,7 +238,7 @@ static inline bool percpu_ref_tryget(struct percpu_ref *ref)
>  static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
>  {
>  	unsigned long __percpu *percpu_count;
> -	int ret = false;
> +	bool ret = false;
>  
>  	rcu_read_lock_sched();
>  
> 
Clean upstream cherrypick, looks sane to me.

Acked-by: Colin Ian King <colin.king at canonical.com>